OAT and ODC expression in salinity challenged mussels

Mytilid mussels (Mytilus spp.) are dominant members of coastal communities in North America and distributions of closely related species are often linked to differential tolerance to environmental stressors, such as salinity. As osmoconformers, mussels initiate cellular and molecular level changes during periods of salinity variation and previous studies of transcriptomic responses to hypoosmotic exposure in blue mussels have suggested differential utilization of the amino acid ornithine. Ornithine catabolism is used to generate glutamate or proline through the activity of ornithine aminotransferase (OAT), or to create putrescine and other polyamines through activity of ornithine decarboxylase (ODC). This study examined species-specific variation in the expression of genes involved in ornithine metabolic pathways for the euryhaline mussel M. trossulus compared to the congeners M. edulis and M. galloprovincialis, which are less tolerant to salinity variation. We found a consistent, small decrease in the expression of ODC during hypoosmotic exposure in all three species, but strong, species-specific increases in OAT expression. During hyperosmotic stress, the patterns of expression of these genes reversed. These patterns suggest that proline or glutamate synthesis is important during low salinity exposure, while polyamine synthesis may be more important during hyperosmotic exposure, which is an indication of differential osmotic stress. These responses were most pronounced in M. galloprovincialis, which showed nearly 18-fold increase in OAT expression during low salinity and a 2-fold increase in ODC expression during high salinity exposure.